A cross-sectional view of a standard thermoelectric couple 10 is shown in FIG. 1. The thermoelectric couple consists of two volumes of semiconductor materials 20 and 30 having dissimilar characteristics, connected electrically in series, and thermally in parallel between a high temperature heat source 14 and a lower temperature heat sink 12. The semiconductor materials are n-type 30 which has more electrons than necessary to complete a perfect molecular lattice and p-type 20 which has an electron deficit or electron vacancies with respect to developing a perfect lattice.
The laws of physics indicate that when such a couple is subjected to a heat flow between the heat source 14 and heat sink 12 an electric potential is developed across the thermoelectric couple. If the couple is connected to an electrical load 16 a current 8 is developed in the direction of the heat flux (or temperature gradient) in the p-type material 20 and in the opposite direction in the n-type material
Characteristics of good semiconductor materials include low thermal conductivities so that a large temperature gradient may be maintained between the heat source and sink as well as high electrical conductivities so that there is little impedance to electron flow.
Electrical conductors 26 provide the series connections for conducting the current through the thermoelectric couple and the electrical load. If the heat source and sink are conductive they must be electrically separated from their surroundings or these conductors 26 by electrical insulators 24.
The extra electrons in the n-type material 30 and the vacancies in the p-type material 20 are acted upon by the heat flux between heat source 14 and heat sink 12 to produce the current 8 which is proportional to the heat flux. The heat flux to the heat source may be supplied by the combustion products from a gas turbine combustor before these products enter the turbine to do expansion work in which case the power from the thermoelectric couple would represent topping cycle work for a combined cycle power plant. If the heat flux to the heat sink is absorbed in a boiler which supplies steam to a steam turbine while the heat source gas goes to a gas turbine a doubly topped gas turbine cycle would be possible.
In practical applications to generate electricity thermoelectric couples 10 are combines in a modular assembly 90 where they are connected electrically in series and thermally in parallel as shown in FIG. 2. The temperature difference across the couples 10 forces the current to make the loop through the p-type material 20 toward the cold junction 12, across a conductor 26, back through the n-type material 30 toward the hot junction 14, across another conductor 26 and through the next couple 10 in the same manner. The resulting build up in voltage is proportional to the number of couples 10 in the module 90.
Modules 90 are available in a variety of sizes, shapes, operating currents, operating voltages, number of couples, and ranges of heat-pumping levels. For large power generation systems, however, where a great deal of heat is generated, grouping thousands of couples 10 in modules 90 to generate electricity would be a very costly and impractical procedure. Conventional thermoelectric couples 10 produce a relatively small amount of electricity per unit volume. Consequently, grouping together the large number of couples 10 required to generate electricity in a large power generation system would be quite inefficient because merely arranging the couples 10 in a systematic manner would be rather difficult. It is, therefore, desirable to take advantage of the benefits of semiconductor materials and provide a configuration of a thermoelectric device for use in a shell and tube heat exchanger of a large power generation system to generate electricity. A shell and tube heat exchanger configuration has a very large heat transfer surface to overall volume ratio.